Ultraviolet light absorbing compounds are effective in sunburn-preventing cosmetics, in preventing yellowing of fibers, resins, etc. and in protecting containers, packing materials and contents therein from ultraviolet light, and the compounds are used in broad fields such as paints, resins, medicines, foodstuffs and cosmetics.
Since short wavelength ultraviolet light (UVC) having a wavelength of not more than about 290 nm is absorbed by the ozone layer surrounding the earth, the ultraviolet light component of sunlight which influences human skin and industrial products is medium wavelength ultraviolet light (UVB) having a wavelength of 290 to 320 nm and long wavelength ultraviolet light (UVA) having a wavelength of 320 to 400 nm.
Hitherto, a large number of UVB absorbing compounds have been known.
For instance, esters of p-dimethylaminobenzoic acid are the compounds which are used in cosmetics and industrial fields as a UVB absorber and have a structure represented by the following formula (I): ##STR1##
Of the above-mentioned compounds, the compound wherein R.sup.1 represents a 2-ethylhexyl group is used as a raw material for cosmetics of broad use in sun-screen agents, etc.
The above-mentioned esters of p-dimethylaminobenzoic acid have been known to be decomposed by light irradiation. See A.E. Mark et al., "Photochemistry and Photobiology", 5, 533-542(1966).
It has been known that the esters of p-dimethylaminobenzoic acid are decomposed by light irradiation and that the discoloration thereof and the deterioration of the ultraviolet light-absorbency thereof are caused as a function of time. Accordingly, there has been a weak point in the chemical stability of the esters for use thereof in combination with the cosmetics, etc. for a long time period. And, although considerations such as coloring the product preliminarily have been paid concerning the discoloration of the esters of p-dimethylaminobenzoic acid, any method for fundamentally settling the deterioration of ultraviolet light absorbency accompanying the decomposition of the esters has not been known.
Although a great number of UVB absorbing compounds have been known to show the same behavior as that of the esters of p-dimethylaminobenzoic acid represented by the formula (I), it has been surprisingly found by the present inventors that, of the above-mentioned UVB absorbing compounds, the esters of p-dimethylaminobenzoic acid of the formula (I), and the esters of p-methoxycinnamic acid of the formula (II): ##STR2## (II) are specifically stabilized by the novel benzylidene compounds of the formula (IV):
On the basis of the above-mentioned finding, the present inventors have completed the present invention.
The present invention is more effective in sunburn-preventing cosmetics, in preventing yellowing of fibers, resins, etc. and in protecting containers, packing materials and the contents therein from ultraviolet lights, and the present invention can be applied in broad fields such as paints, resins, medicines, foodstuffs and cosmetics. Since the above-mentioned novel compounds of the formula (IV) have the UVA absorbency, in the case where the compound of the formula (IV) is used in combination with the above-mentioned UVB absorbing compounds according to the present invention, not only these UVB absorbing compounds are stabilized but also UVA is absorbed, and accordingly, the present invention is particularly applied advantageously in sunburn-preventing cosmetics.